AbstractDensity functional theory (DFT) calculations were carried out for reactions forming (A) phenolphthalein and (B) fluorescein. M06‐2X/6‐311G** and M06‐2X/6‐311++G(2d,p) with SCRF=PCM were applied to the search for elementary processes. Reactant models are (A) phthalic anhydride + (phenol)2 + H2SO4 and (B) phthalic anhydride + (resorcinol)2 + H2SO4, respectively. For (A), the acid catalyzed ortho and para phenol additions to phthalic anhydride were compared. While the ortho addition is more favorable than the para one, the ortho adduct involves the strong O2(HO)S–O−....H–O hydrogen bond and the reaction progress was blocked. In the para route, five elementary processes were obtained to arrive at the product, phenolphthalein. For (B), there are two reaction channels from the phenolphthalein‐like intermediate. The first channel is the direct H2O elimination to the lactoid tautomer of fluorescein. This channel has the significantly large activation energy and is unlikely. The second one includes three intermediates with the open form of the γ‐butyrolactone ring. Nine elementary processes were obtained for the fluorescein formation. A carbocation intermediate was found to be the key one for affording three tautomers (quinoid, zwitterionic, and lactoid) of fluorescein.